Working faces to be dug in by tunnel boring machines are divided into the ground having a not-disintegrative geological feature and the ground having a disintegrative geological feature. When digging in the ground having a disintegrative geological feature, a method called a slurry pressure technique is generally used. According to this method, a water-tight chamber enclosed by a partition wall is formed on the back side of a cutter disk, and compressed water is supplied to the chamber to fill it with water under pressure, thereby preventing a collapse of the working face with the water pressure of the compressed water. Further, the earth excavated by the cutter disk is collected in a lower portion of the chamber, and then discharged along with the compressed water rearwardly of the partition wall under the pressure of the compressed water in the chamber through a discharge pipe connected to the partition wall.
Such a slurry pressure technique is extremely complicated and expensive in equipment because sealing mechanisms are required between a body of the tunnel boring machine and the surrounding natural ground and between the exterior and interior of the boring machine body for keeping water-tight the chamber on the back side of the cutter disk. For that reason, the slurry pressure technique is used only when digging in the ground having a disintegrative geological feature, and a non-pressure technique is generally used when digging in the ground having a non-disintegrative geological feature.
As a tunnel boring machine operable in a non-pressure manner to dig into ground having a non-disintegrative geological feature, there is a known conventional one wherein carrying-out means such as a belt conveyor or a screw conveyor is disposed on the back side of a cutter disk. The earth excavated by the cutter disk is discharged rearward by the carrying-out means.
Further, to make smaller the size of the carrying-out means and reduce frequency in occurrence of troubles thereof, JP, Y, 4-49274 and JP, B, 4-11720 propose a tunnel boring machine using a jet pump as carrying-out means. According to this proposal, a hopper is disposed in a lower portion of a chamber formed between a cutter disk and a partition wall, and the earth excavated by the cutter disk is collected in the hopper. The jet pump having an earth take-in port formed in its casing is attached to a bottom portion of the hopper, and a discharge pipe is connected to a casing outlet of the jet pump. Compressed water is supplied to the jet pump through a piping from a supply pump provided rearwardly of the boring machine. The compressed water is accelerated by a nozzle of the jet pump, and then depressurized in a throat portion downstream of the earth take-in port to produce a negative pressure. With a water flow developed under the negative pressure, the earth in the hopper is discharged through the earth take-in port and then the discharge pipe.